Work vehicle and control method for work vehicle

ABSTRACT

A work vehicle includes an engine, a traveling pump driven by the engine, a hydraulic circuit connected to the traveling pump, a traveling motor connected to the traveling pump via the hydraulic circuit, a rotational speed sensor configured to output a signal indicative of engine speed a hydraulic pressure sensor configured to output a signal indicative of hydraulic pressure of the hydraulic circuit, a controller. The controller receives the signals from the rotational speed sensor and the hydraulic pressure sensor. The controller determines whether the engine is in an over-rotation state based on engine speed. The controller determines an upper limit value of a displacement of the traveling pump based on hydraulic pressure of the hydraulic circuit. The controller limits the displacement of the traveling pump to the upper limit value or less upon determining that the engine is in the over-rotation state.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National stage application of International Application No. PCT/JP2018/020534, filed on May 29, 2018. This U.S. National stage application claims priority under 35 U.S.C. § 119(a) to Japanese Patent Application No. 2017-125375, filed in Japan on Jun. 27, 2017, the entire contents of which are hereby incorporated herein by reference.

BACKGROUND Field of the Invention

The present invention relates to a work vehicle and a control method for a work vehicle.

Background Information

Some work vehicles are equipped with a hydrostatic transmission. The hydrostatic transmission includes a traveling pump, a traveling motor, and a hydraulic circuit connecting the traveling pump and the traveling motor. The traveling pump is driven by the engine and discharges hydraulic fluid. The hydraulic fluid discharged from the traveling pump is supplied to the traveling motor via the hydraulic circuit. The traveling motor is driven by hydraulic fluid from the traveling pump. The traveling motor is connected to a traveling device of the work vehicle, and the work vehicle travels by driving the traveling motor. In the hydrostatic transmission, the transmission gear ratio can be controlled by controlling the displacement of the traveling pump and the displacement of the traveling motor.

In work vehicles, engine brakes are applied when going down a steep slope. In that case, if the acceleration force of the vehicle by gravity is strong, there is a possibility that the engine speed may be excessive (hereinafter referred to as “over-rotation state”).

Therefore, in Japanese Patent Laid-Open No. 2012-57761, in the work vehicle, the transmission gear ratio of the hydrostatic transmission is reduced by reducing the displacement of the traveling motor or increasing the displacement of the traveling pump so that Increase in engine speed can be suppressed.

SUMMARY

However, as described above, when the transmission gear ratio is reduced by reducing the displacement of the traveling motor or increasing the displacement of the traveling pump, the braking force of the vehicle by the engine brake is weakened. Therefore, in order to suppress an increase in the vehicle speed, the operator needs to operate the brake pedal together.

An object of the present invention is to prevent over-rotation of an engine and to suppress a decrease in braking force due to an engine brake in a work vehicle.

A work vehicle according to the first aspect includes an engine, a traveling pump, a hydraulic circuit, a traveling motor, a rotational speed sensor, a hydraulic pressure sensor, and a controller. The traveling pump is driven by the engine. The hydraulic circuit is connected to the traveling pump. The traveling motor is connected to the traveling pump via the hydraulic circuit. The rotational speed sensor outputs a signal indicative of the engine speed. The hydraulic pressure sensor outputs a signal indicative of the hydraulic pressure of the hydraulic circuit.

The controller receives signals from the rotational speed sensor and the hydraulic pressure sensor. The controller determines whether the engine is in an over-rotation state based on the engine speed. The controller determines the upper limit value of the displacement of the traveling pump according to the hydraulic pressure of the hydraulic circuit. The controller limits the displacement of the traveling pump to the upper limit value or less upon determining that the engine is in the over-rotation state.

In the work vehicle according to the present aspect, the upper limit value of the displacement of the traveling pump is determined according to the hydraulic pressure of the hydraulic circuit, and the displacement of the traveling pump is limited to less than or equal to the upper limit value when the engine is in the over-rotation state. The engine reverse drive torque transmitted from the traveling pump to the engine increases as the displacement of the traveling pump increases. Therefore, by limiting the displacement of the traveling pump to the upper limit value or less, the engine reverse drive torque can be suppressed to a small value. Thereby, an increase in engine speed can be suppressed to prevent over-rotation.

Further, the engine reverse drive torque transmitted from the traveling pump to the engine changes in accordance with the hydraulic pressure of the hydraulic circuit. Therefore, the engine reverse drive torque can be appropriately suppressed by determining the upper limit value of the displacement of the traveling pump according to the hydraulic pressure of the hydraulic circuit. Further, since the reduction of the transmission ratio can be suppressed, the reduction of the braking force due to the engine brake can be suppressed.

The controller may decrease the upper limit value when the engine is in the over-rotation state. In this case, by reducing the upper limit value, the displacement of the traveling pump is reduced, whereby the engine reverse drive torque is reduced. Thereby, the engine speed can be reduced to prevent over-rotation. In addition, since the reduction of the displacement of the traveling pump increases the transmission ratio, the reduction of the braking force by the engine brake can be suppressed.

The controller may decrease the upper limit value in response to an increase in hydraulic pressure of the hydraulic circuit. In this case, the engine reverse drive torque increases in response to the increase in the displacement of the traveling pump and the increase in the hydraulic pressure of the hydraulic circuit. Therefore, by reducing the upper limit value of the traveling pump according to the increase in the hydraulic pressure of the hydraulic circuit, the increase of the engine reverse drive torque can be appropriately suppressed.

The controller may determine the upper limit value of the displacement of the traveling pump so that the engine reverse drive torque transmitted from the traveling pump to the engine does not exceed a predetermined allowable value. In this case, the engine reverse drive torque can be suppressed to the predetermined allowable value or less.

The controller may determine that the engine is in the over-rotation state when the engine speed is equal to or greater than a predetermined first threshold. The allowable value may be a value of the engine brake torque when the engine speed is a second threshold equal to or higher than the first threshold. In this case, before the engine reverse drive torque rises to the allowable value, it is possible to determine with a margin that the engine is in the over-rotation state. Thereby, sudden change of the displacement of the traveling pump can be suppressed.

A method according to the second aspect is a method executed by a controller for controlling a work vehicle. The work vehicle includes an engine, a traveling pump, a hydraulic circuit, and a traveling motor. The traveling pump is driven by the engine. The hydraulic circuit is connected to the traveling pump. The traveling motor is connected to the traveling pump via the hydraulic circuit. The method according to the present aspect comprises the following processing.

The first process is to receive a signal indicative of the engine speed. The second process is to receive a signal indicative of the hydraulic pressure of the hydraulic circuit. The third process is to determine whether the engine is in an over-rotation state based on the engine speed. The fourth process is to determine the upper limit value of the displacement of the traveling pump according to the hydraulic pressure of the hydraulic circuit. The fifth process is to limit the displacement of the traveling pump to the upper limit value or less when the engine is in the over-rotation state.

In the method according to the present aspect, the upper limit value of the displacement of the traveling pump is determined according to the hydraulic pressure of the hydraulic circuit, and when the engine is in the over-rotation state, the displacement of the traveling pump is limited to the upper limit value or less. The engine reverse drive torque transmitted from the traveling motor to the engine increases as the displacement of the traveling pump increases. Therefore, by limiting the displacement of the traveling pump to the upper limit value or less, the engine reverse drive torque can be suppressed to a small value. Thereby, an increase in engine speed can be suppressed to prevent over-rotation.

Further, the engine reverse drive torque transmitted from the traveling pump to the engine changes in accordance with the hydraulic pressure of the hydraulic circuit. Therefore, the engine reverse drive torque can be appropriately suppressed by determining the upper limit value of the displacement of the traveling pump according to the hydraulic pressure of the hydraulic circuit. Further, since the reduction of the transmission ratio can be suppressed, the reduction of the braking force due to the engine brake can be suppressed.

According to the present invention, it is possible to prevent over-rotation of the engine and to suppress a decrease in braking force due to the engine brake.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of a work vehicle according to an embodiment.

FIG. 2 is a block diagram showing a configuration of a drive system of the work vehicle.

FIG. 3 is a block diagram showing a configuration of a control system of the work vehicle.

FIG. 4 is a diagram showing vehicle speed-traction force characteristics of the work vehicle.

FIG. 5 is a view showing an example of a vehicle speed-traction force characteristic which is changed according to the operation of an accelerator.

FIG. 6 is a flow chart illustrating processing executed by the controller.

FIG. 7 is a diagram showing processing for determining a target vehicle speed from the amount of operation of an accelerator.

FIG. 8 is a diagram showing vehicle speed-input horsepower characteristics of the work vehicle.

FIG. 9 is a diagram showing processing for determining a target input horsepower in stalling;

FIG. 10 is a diagram showing processing for determining the target input horsepower in a low vehicle speed range and a middle vehicle speed range.

FIG. 11 is a diagram showing processing for determining the target input horsepower in a high vehicle speed range.

FIG. 12 is a diagram showing processing for determining the target input horsepower at a transition time.

FIG. 13 is a diagram showing a process for determining a target rotational speed of an engine.

FIG. 14A and FIG. 14B are diagrams showing processing for determining target displacements of a traveling pump and a traveling motor.

FIG. 15 is a flow chart showing a process executed by the controller to suppress over-rotation of the engine.

FIG. 16 is a view showing a relationship between an engine speed and an engine brake torque.

DETAILED DESCRIPTION OF EMBODIMENT(S)

Hereinafter, a work vehicle 1 according to an embodiment of the present invention will be described using the drawings. FIG. 1 is a side view of the work vehicle 1. The work vehicle 1 is a wheel loader. The work vehicle 1 includes a vehicle body 2, a work implement 3, a plurality of traveling wheels 4, and a cab 5. The work implement 3 is attached to the front of the vehicle body 2. The work implement 3 includes a boom 11, a bucket 12, a lift cylinder 13 and a bucket cylinder 14.

The boom 11 is rotatably attached to the vehicle body 2. The boom 11 is driven by the lift cylinder 13. The bucket 12 is rotatably attached to the boom 11. The bucket 12 is moved up and down by the bucket cylinder 14. The cab 5 is disposed on the vehicle body 2. The plurality of traveling wheels 4 are rotatably attached to the vehicle body 2.

FIG. 2 is a block diagram showing a configuration of a drive system mounted on work vehicle 1. The work vehicle 1 includes an engine 21, a work implement pump 22, and a hydrostatic transmission (hereinafter referred to as “HST”) 23. The engine 21 is, for example, a diesel engine.

A fuel injector 24 is connected to the engine 21. The fuel injector 24 controls the amount of fuel injection to the engine 21 to control the output torque of the engine 21 (hereinafter referred to as “engine torque”) and the rotational speed. The actual rotational speed of the engine 21 is detected by an engine speed sensor 25. The engine speed sensor 25 outputs a signal indicative of the actual rotational speed of the engine 21.

The work implement pump 22 is connected to the engine 21. The work implement pump 22 is driven by the engine 21 to discharge hydraulic fluid. The hydraulic fluid discharged from the work implement pump 22 is supplied to the lift cylinder 13 via the work implement hydraulic circuit 26. Thereby, the work implement 3 is driven. The discharge pressure of the work implement pump 22 is detected by a work implement pump pressure sensor 27. The work implement pump pressure sensor 27 outputs a signal indicative of the discharge pressure of the work implement pump 22.

The work implement pump 22 is a variable displacement hydraulic pump. A pump displacement control device 28 is connected to the work implement pump 22. The pump displacement control device 28 controls the displacement of the work implement pump 22. The pump displacement control device 28 includes a servo piston 28 a and a pump control valve 28 b. The servo piston 28 a is connected to the work implement pump 22. The displacement of the work implement pump 22 is changed by the servo piston 28 a changing the tilt angle of the work implement pump 22. The pump control valve 28 b controls the hydraulic pressure supplied to the servo piston 28 a to control the operation of the servo piston 28 a. The work implement pump 22 may be a fixed displacement hydraulic pump.

A work implement control valve 30 is disposed in the work implement hydraulic circuit 26. The work implement control valve 30 controls the flow rate of the hydraulic fluid supplied to the lift cylinder 13 in accordance with the pilot pressure applied to the work implement control valve 30. Although not shown, the work implement control valve 30 may control the flow rate of the hydraulic fluid supplied to the bucket cylinder 14. The flow rate of the hydraulic fluid means the amount of hydraulic fluid supplied per unit time. The work implement control valve 30 is not limited to the hydraulic pilot-operated control valve, and may be an electromagnetic control valve that is electrically controlled.

The HST 23 includes a traveling pump 31, a driving hydraulic circuit 32, and a traveling motor 33. The traveling pump 31 is connected to the engine 21. The traveling pump 31 discharges hydraulic fluid by being driven by the engine 21. The traveling pump 31 is a variable displacement hydraulic pump. The hydraulic fluid discharged from the traveling pump 31 is sent to the traveling motor 33 through the driving hydraulic circuit 32.

The driving hydraulic circuit 32 connects the traveling pump 31 and the traveling motor 33. The driving hydraulic circuit 32 includes a first drive circuit 32 a and a second drive circuit 32 b. The first drive circuit 32 a connects one port of the traveling pump 31 and one port of the traveling motor 33. The second drive circuit 32 b connects the other port of the traveling pump 31 and the other port of the traveling motor 33. The traveling pump 31, the traveling motor 33, the first drive circuit 32 a, and the second drive circuit 32 b constitute a closed circuit.

The traveling motor 33 is driven in one direction (for example, the forward direction) by being supplied from the traveling pump 31 to the traveling motor 33 via the first drive circuit 32 a. In this case, the hydraulic fluid returns from the traveling motor 33 to the traveling pump 31 via the second drive circuit 32 b. Further, the hydraulic fluid is supplied from the traveling pump 31 to the traveling motor 33 via the second drive circuit 32 b, whereby the traveling motor 33 is driven in the other direction (for example, the reverse direction). In this case, the hydraulic fluid returns from the traveling motor 33 to the traveling pump 31 via the first drive circuit 32 a.

The driving hydraulic circuit 32 is provided with a drive circuit pressure sensor 34. The drive circuit pressure sensor 34 detects the pressure of the hydraulic fluid supplied to the traveling motor 33 via the first drive circuit 32 a or the second drive circuit 32 b. Specifically, the drive circuit pressure sensor 34 includes a first circuit pressure sensor 34 a and a second circuit pressure sensor 34 b.

The first circuit pressure sensor 34 a detects the hydraulic pressure of the first drive circuit 32 a. The second circuit pressure sensor 34 b detects the hydraulic pressure of the second drive circuit 32 b. The first circuit pressure sensor 34 a outputs a signal indicative of the hydraulic pressure of the first drive circuit 32 a. The second circuit pressure sensor 34 b outputs a signal indicative of the hydraulic pressure of the second drive circuit 32 b.

The traveling motor 33 is a variable displacement hydraulic motor. The traveling motor 33 is driven by the hydraulic fluid discharged from the traveling pump 31 to generate a driving force for traveling. A motor displacement control device 35 is connected to the traveling motor 33. The motor displacement control device 35 controls the displacement of the traveling motor 33. The motor displacement control device 35 includes a motor cylinder 35 a and a motor control valve 35 b.

The motor cylinder 35 a is connected to the traveling motor 33. The motor cylinder 35 a is driven by hydraulic pressure to change the tilt angle of the traveling motor 33. The motor control valve 35 b is an electromagnetic control valve controlled based on a command signal input to the motor control valve 35 b. The motor control valve 35 b operates the motor cylinder 35 a to change the displacement of the traveling motor 33.

The traveling motor 33 is connected to the drive shaft 37. The drive shaft 37 is connected to the traveling wheels 4 described above via an axle (not shown). The rotation of the traveling motor 33 is transmitted to the traveling wheels 4 via the drive shaft 37. Thereby, the work vehicle 1 travels.

The work vehicle 1 is provided with a vehicle speed sensor 36. The vehicle speed sensor 36 detects the vehicle speed. The vehicle speed sensor 36 outputs a signal indicative of the vehicle speed. For example, the vehicle speed sensor 36 detects the vehicle speed by detecting the rotational speed of the drive shaft 37.

HST 23 includes a charge pump 38 and a charge circuit 39. The charge pump 38 is a fixed displacement hydraulic pump. The charge pump 38 is connected to the engine 21. The charge pump 38 is driven by the engine 21 to supply hydraulic fluid to the driving hydraulic circuit 32.

The charge circuit 39 is connected to the charge pump 38. The charge circuit 39 is connected to the first drive circuit 32 a via a first check valve 41. The charge circuit 39 is connected to the second drive circuit 32 b via a second check valve 42.

The charge circuit 39 is connected to the first drive circuit 32 a via a first relief valve 43. The first relief valve 43 is opened when the hydraulic pressure of the first drive circuit 32 a becomes larger than a predetermined relief pressure. The charge circuit 39 is connected to the second drive circuit 32 b via a second relief valve 44. The second relief valve 44 is opened when the hydraulic pressure of the second drive circuit 32 b becomes larger than a predetermined relief pressure.

A charge relief valve 40 is provided in the charge circuit 39. The charge relief valve 40 is opened when the hydraulic pressure of the charge circuit 39 becomes larger than a predetermined relief pressure. Thereby, the hydraulic pressure of the charge circuit 39 is limited so as not to exceed the predetermined relief pressure.

A pump displacement control device 45 is connected to the traveling pump 31. The pump displacement control device 45 controls the displacement of the traveling pump 31. The displacement of the hydraulic pump means the discharge amount (cc/rev) of hydraulic fluid per one rotation. Further, the pump displacement control device 45 controls the discharge direction of the traveling pump 31. The pump displacement control device 45 includes a pump control cylinder 46 and a pump control valve 47.

The pump control cylinder 46 is connected to the traveling pump 31. The pump control cylinder 46 is hydraulically driven to change the tilt angle of the traveling pump 31. Thus, the pump control cylinder 46 changes the displacement of the traveling pump 31. The pump control cylinder 46 is connected to the charge circuit 39 via a pump pilot circuit 48.

The pump control valve 47 is an electromagnetic control valve controlled based on a command signal input to the pump control valve 47. The pump control valve 47 switches the supply direction of the hydraulic fluid to the pump control cylinder 46. The pump control valve 47 switches the discharge direction of the traveling pump 31 by switching the supply direction of the hydraulic fluid to the pump control cylinder 46. Thereby, the driving direction of the traveling motor 33 is changed, and the forward and reverse of the work vehicle 1 are switched.

The pump control valve 47 also controls the pressure of hydraulic fluid supplied to the pump control cylinder 46 via the pump pilot circuit 48. Specifically, the pump control valve 47 adjusts the tilt angle of the traveling pump 31 by changing the pressure of the hydraulic fluid supplied to the pump control cylinder 46. Thereby, the displacement of the traveling pump 31 is controlled.

The pump pilot circuit 48 is connected to the hydraulic fluid tank via the cut-off valve 52. The pilot ports of the cut-off valve 52 are connected to the first drive circuit 32 a and the second drive circuit 32 b via a shuttle valve 53. The shuttle valve 53 introduces the larger one of the hydraulic pressure of the first drive circuit 32 a and the hydraulic pressure of the second drive circuit 32 b (hereinafter referred to as “drive circuit pressure”) into the pilot port of the cut-off valve 52.

The cut-off valve 52 causes the pump pilot circuit 48 to communicate with the hydraulic fluid tank when the drive circuit pressure becomes equal to or higher than a predetermined cut-off pressure. As a result, the hydraulic pressure of the pump pilot circuit 48 is reduced, whereby the displacement of the traveling pump 31 is reduced. As a result, an increase in drive circuit pressure can be suppressed.

FIG. 3 is a schematic view showing a control system of the work vehicle 1. As shown in FIG. 3, the work vehicle 1 includes an accelerator 61, an FR operating member 62, and a shift operating member 63. The accelerator 61, the FR operating member 62, and the shift operating member 63 are disposed to be operable by the operator. The accelerator 61, the FR operating member 62, and the shift operating member 63 are disposed in the cab 5.

The accelerator 61 is, for example, an accelerator pedal. However, the accelerator 61 may be another member such as a lever or a switch. The accelerator 61 is connected to an accelerator operation sensor 64. The accelerator operation sensor 64 is, for example, a position sensor that detects the position of the accelerator 61. The accelerator operation sensor 64 outputs a signal indicative of an operation amount of the accelerator 61 (hereinafter referred to as “accelerator operation amount”). The accelerator operation amount is, for example, represented by a ratio when the state in which the accelerator 61 is operated fully open is 100%. As described later, the operator can control the vehicle speed and the traction force by adjusting the accelerator operation amount.

The FR operating member 62 is, for example, an FR lever. However, the FR operating member 62 may be another member such as a switch. The FR operating member 62 is switched between a forward position, a reverse position and a neutral position. The FR operating member 62 is connected to the FR operation sensor 65. The FR operation sensor 65 is, for example, a position sensor that detects the position of the FR operating member 62. The FR operation sensor 65 outputs a signal indicative of the position of the FR operating member 62. The operator can switch between forward and reverse of the work vehicle 1 by operating the FR operating member 62.

The shift operating member 63 is, for example, a dial switch. However, the shift operating member 63 may be another member such as a lever. The shift operating member 63 is connected to the shift operation sensor 66. The shift operation sensor 66 is, for example, a position sensor that detects the position of the shift operating member 63 (hereinafter referred to as “shift position”). The shift operation sensor 66 outputs a signal indicative of the shift position. The shift position includes, for example, the positions of first to fourth speeds. However, the shift position may include a position faster than the fourth speed. Alternatively, the shift position may be from the first speed to a position slower than the fourth speed.

FIG. 4 is a diagram showing a vehicle speed-traction force characteristic of the work vehicle 1. As shown in FIG. 4, the operator can select a shift pattern (L_1st to L_4th) that defines the maximum vehicle speed by operating the shift operating member 63.

The work vehicle 1 includes a work implement operating member 67. The work implement operating member 67 is, for example, a work implement lever. However, the work implement operating member 67 may be another member such as a switch. A pilot pressure corresponding to the operation of the work implement operating member 67 is applied to the work implement control valve 30. The work implement operating member 67 is connected to a work implement operation sensor 68. The work implement operation sensor 68 is, for example, a pressure sensor. The work implement operation sensor 68 detects the amount of operation of the work implement operating member 67 (hereinafter referred to as “work implement operation amount”) and the operation direction, and outputs signals indicative of the work implement operation amount and the operation direction. When the work implement control valve 30 is not a pressure proportional control valve but an electromagnetic proportional control valve, the work implement operation sensor 68 is a position sensor that electrically detects the position of the work implement operating member 67. The operator can operate the work implement 3 by operating the work implement operating member 67. For example, the operator can raise or lower the bucket 12 by operating the work implement operating member 67.

The work vehicle 1 includes an input device 69. The input device 69 is, for example, a touch panel. However, the input device 69 is not limited to the touch panel, and may be another device such as a switch. The operator can perform various settings of the work vehicle 1 by operating the input device 69. For example, the input device 69 can set traction control. As shown in FIG. 4, The traction control is a feature that allows the maximum traction force to be selected from a plurality of traction levels.

The plurality of traction levels include a first level and a second level. At the first level, the maximum traction force is limited to a value less than the normal maximum traction force where traction control is disabled. At the second level, the maximum traction force is limited to a value smaller than the maximum traction force at the first level.

In FIG. 4, L_max represents the vehicle speed-traction force characteristic of the work vehicle 1 at the normal time when the traction control is disabled. L_TC1 shows the vehicle speed-traction force characteristic in the traction control at the first level. L_TC2 shows the vehicle speed-traction force characteristic in the traction control at the second level.

As shown in FIG. 3, the work vehicle 1 includes a storage device 71 and a controller 72. The storage device 71 includes, for example, a memory and an auxiliary storage device. The storage device 71 may be, for example, a RAM or a ROM. The storage device 71 may be a semiconductor memory or a hard disk. The storage device 71 is an example of a non-transitory computer readable recording medium. The storage device 71 stores computer instructions that can be executed by a processor and control the work vehicle 1.

The controller 72 includes, for example, a processor such as a CPU. The controller 72 is communicably connected to the above-described sensor, the input device 69, and the storage device 71. The controller 72 is communicably connected to the various sensors described above, the input device 69, and the storage device 71 in a wired or wireless manner. The controller 72 acquires various data by receiving signals from the sensor, the input device 69, and the storage device 71. The controller 72 is programmed to control the work vehicle 1 based on the acquired data. The controller 72 may be configured by a plurality of controllers separate from one another.

The controller 72 is communicably connected to the control valves 35 b and 47 and the fuel injector 24 described above in a wired or wireless manner. The controller 72 controls the control valves 35 b and 47 and the fuel injector 24 by outputting command signals to the control valves 35 b and 47 and the fuel injector 24.

Specifically, the controller 72 controls the engine torque and the engine speed by outputting a command signal to the fuel injector 24. The controller 72 controls the displacement of the traveling motor 33 by outputting a command signal to the motor control valve 35 b. The controller 72 controls the displacement of the traveling pump 31 by outputting a command signal to the pump control valve 47. The controller 72 controls the transmission ratio of the HST 23 by controlling the displacement of the traveling pump and the displacement of the traveling motor so that the vehicle speed-traction force characteristic as shown in FIGS. 4 and 5 is realized.

Next, control of the work vehicle 1 by the controller 72 will be described. In the work vehicle 1 according to the present embodiment, the controller 72 determines a target rotational speed of the engine 21 (hereinafter referred to as “target engine speed”) based on the accelerator operation amount and the work implement operation amount. The operator can increase the engine speed by operating the work implement operating member 67 without operating the accelerator 61. Further, even if the work implement operating member 67 and the accelerator 61 are simultaneously operated, the traveling performance of the vehicle can be adjusted by the accelerator 61 without being affected by the operation of the work implement operating member 67.

FIG. 5 is a view showing an example of a vehicle speed-traction force characteristic which is changed according to the operation of the accelerator 61 by the operator. In FIG. 5, T100 represents the vehicle speed-traction force characteristic when the accelerator operation amount is 100%. T80 indicates the vehicle speed-traction force characteristic when the accelerator operation amount is 80%. T60 shows the vehicle speed-traction force characteristic when the accelerator operation amount is 60%. In the work vehicle 1 according to the present embodiment, even when the work implement operating member 67 and the accelerator 61 are simultaneously operated, traveling performance (vehicle speed-traction force characteristic) can be obtained according to the accelerator operation amount.

The processing executed by the controller 72 will be described below. FIG. 6 is a flowchart showing the process executed by the controller 72. In the following description, control when the work vehicle 1 moves forward will be described. However, similar control may be performed when the work vehicle 1 moves backward.

As shown in FIG. 6, in S101, the controller 72 acquires an accelerator operation amount. The controller 72 acquires the accelerator operation amount in accordance with a signal from the accelerator operation sensor 64.

In step S102, the controller 72 determines a target vehicle speed. The controller 72 determines the target vehicle speed from the accelerator operation amount. FIG. 7 shows a process for determining the target vehicle speed from the accelerator operation amount.

As shown in FIG. 7, in step S201, the controller 72 determines a target reference vehicle speed from the accelerator operation amount and the shift position. The target reference vehicle speed is a vehicle speed that is set as the target achieved vehicle speed when the work vehicle 1 travels on a flat ground. The storage device 71 stores reference vehicle speed data D1 that defines the relationship between the accelerator operation amount and the target reference vehicle speed. In the reference vehicle speed data D1, the target reference vehicle speed is increased according to the increase of the accelerator operation amount. The reference vehicle speed data D1 defines the relationship between the accelerator operation amount and the target reference vehicle speed for each shift position. In the reference vehicle speed data D1, even if the accelerator operation amount is the same, the target reference vehicle speed increases as the shift position is on the high speed side. The controller 72 refers to the reference vehicle speed data D1 to determine the target reference vehicle speed corresponding to the accelerator operation amount and the shift position.

In step S202, the controller 72 calculates the vehicle speed deviation. The vehicle speed deviation is the difference between the target reference vehicle speed and the actual vehicle speed. In step S203, the controller 72 calculates a target acceleration. The controller 72 calculates the target acceleration from the vehicle speed deviation and the accelerator operation amount. Specifically, the controller 72 calculates the target acceleration corresponding to the vehicle speed deviation with reference to the acceleration data D5. The acceleration data D5 defines the relationship between the vehicle speed deviation and the target acceleration. In the acceleration data D5, the target acceleration decreases as the vehicle speed deviation increases. The controller 72 changes the acceleration data D5 in accordance with the accelerator operation amount. The controller 72 changes the acceleration data D5 such that the target acceleration increases as the accelerator operation amount increases, although the vehicle speed deviation is the same. In addition, that the vehicle speed deviation is negative means that the work vehicle 1 is accelerating. That the vehicle speed deviation is positive means that the work vehicle 1 is decelerating. A positive value for the target acceleration means acceleration, and a negative value for the target acceleration means deceleration.

In step S204, the controller 72 calculates a target speed change amount from the target acceleration. The controller 72 multiplies the target acceleration by the calculation cycle of the controller 72 to calculate the target speed change amount.

In step S205 and step S206, the controller 72 adds the target speed change amount to the actual vehicle speed. In step S207, the controller 72 selects the smaller one (first target vehicle speed) of the target reference vehicle speed and the value obtained by adding the target speed change amount to the actual vehicle speed. In step S208, the controller 72 selects the larger one (second target vehicle speed) of the target reference vehicle speed and the value obtained by adding the target speed change amount to the actual vehicle speed.

In step S209, the controller 72 determines the target vehicle speed according to whether the work vehicle 1 is accelerating or decelerating. The controller 72 determines that the work vehicle 1 is accelerating when the actual vehicle speed is lower than the target reference speed. Further, when the actual vehicle speed is larger than the target reference speed, the controller 72 determines that the work vehicle 1 is decelerating. The controller 72 determines the first target vehicle speed as the target vehicle speed during acceleration, and determines the second target vehicle speed as the target vehicle speed during deceleration. When the target vehicle speed is a negative value, the controller 72 sets the target vehicle speed to zero.

Next, as shown in FIG. 6, in step S103, the controller 72 determines the target input horsepower to the HST 23. The target input horsepower to the HST 23 means the horsepower distributed to the HST 23 of the output horsepower of the engine 21. The controller 72 determines the target input horsepower from the accelerator operation amount.

FIG. 8 is a diagram showing a vehicle speed-HST input horsepower characteristic of the work vehicle 1 according to the present embodiment. In FIG. 8, H100 represents the vehicle speed-HST input horsepower characteristic when the accelerator operation amount is 100%. H80 represents the vehicle speed-HST input horsepower characteristic when the accelerator operation amount is 80%. H60 shows the vehicle speed-HST input horsepower characteristic when the accelerator operation amount is 60%.

As shown in FIG. 8, the controller 72 determines the target input horsepower to the HST 23 from the accelerator operation amount so that traveling performance (vehicle speed-HST input horsepower characteristic) corresponding to the accelerator operation amount can be obtained. The controller 72 determines the target input horsepower to the HST 23 in stalling (R_stall), the low vehicle speed range (R_low), the middle vehicle speed range (R_mid), and the high vehicle speed range (R_high) according to the target vehicle speed.

FIG. 9 is a diagram showing processing for determining the target input horsepower to the HST 23 in stalling. As shown in FIG. 9, in step S301, the controller 72 determines a target traction force in stalling from the accelerator operation amount. The storage device 71 stores target traction data D2 that defines the relationship between the accelerator operation amount and the target traction force in stalling.

The target traction data D2 defines the relationship between the accelerator operation amount and the target traction force in stalling at the normal time when the above-described functions such as traction control are not executed. The target traction data D2 defines the target traction force in stalling for the accelerator operation amount in the range of 0 to 100%. In the target traction data D2, the target traction force increases as the accelerator operation amount increases. The controller 72 refers to the target traction data D2 to determine the target traction force in stalling corresponding to the accelerator operation amount.

In step S302, the controller 72 determines the target traction force in stalling at each traction level by multiplying the target traction force in stalling determined in step S301 by the ratio according to the traction level. The ratio is 1 under the normal time where the traction control is not performed.

In step S303, the controller 72 converts the target traction force in stalling determined in step S302 into a target motor torque. The controller 72 calculates the target motor torque by multiplying the target traction force by a predetermined conversion factor and dividing by the transmission machine efficiency. The predetermined conversion factor is a factor for converting the traction force of the work vehicle 1 into a torque at the output shaft of the HST 23. The transmission machine efficiency is the transmission efficiency from the output shaft of the HST 23 to the traveling wheels 4.

In step S304, the controller 72 determines a target HST differential pressure from the target motor torque. The HST differential pressure is a difference between the hydraulic pressure of the first drive circuit 32 a and the hydraulic pressure of the second drive circuit 32 b. The controller 72 calculates the target HST differential pressure by dividing the target motor torque by the maximum displacement of the traveling motor 33 and dividing it by the torque efficiency of the traveling motor 33.

In step S305, the controller 72 determines the target flow rate of the traveling pump 31 from the target HST differential pressure. The storage device 71 stores target flow rate data D3 that defines the relationship between the target HST differential pressure in stalling and the target flow rate of the traveling pump 31. In the target flow rate data D3, the target flow rate of the traveling pump 31 increases according to the increase of the target HST differential pressure. The controller 72 determines the target flow rate of the traveling pump 31 corresponding to the target HST differential pressure with reference to the target flow rate data D3.

In step S306, the controller 72 determines the target input horsepower to the HST 23 in stalling from the target HST differential pressure and the target flow rate of the traveling pump 31. The controller 72 multiplies the target HST differential pressure by the target flow rate of the traveling pump 31 and divides it by the pump torque efficiency to determine the target input horsepower to the HST 23 in stalling.

FIG. 10 is a diagram showing processing for determining the target input horsepower to the HST 23 in the low vehicle speed range and the middle vehicle speed range. As shown in FIG. 10, in step S401, the controller 72 determines a target traveling horsepower from the target traction force in stalling and the target vehicle speed. The controller 72 determines the target traveling horsepower by multiplying the target traction force in stalling by the target vehicle speed and dividing by the transmission efficiency. The transmission efficiency is a transmission efficiency from the input shaft of the HST 23 to the traveling wheels 4.

In step S402, the controller 72 determines the target input horsepower to the HST 23 in the low vehicle speed range from the target traveling horsepower and the target input horsepower in stalling. The controller 72 determines the target input horsepower to the HST 23 in the low vehicle speed range by adding the target traveling horsepower to the target input horsepower in stalling.

In step S403, the controller 72 determines the target input horsepower to the HST 23 in the middle vehicle speed range from the accelerator operation amount. The storage device 71 stores target input horsepower data D4 that defines the relationship between the accelerator operation amount and the target input horsepower to the HST 23. In the target input horsepower data D4, the target input horsepower increases as the accelerator operation amount increases. The controller 72 refers to the target input horsepower data D4 to determine the target input horsepower in the middle vehicle speed range corresponding to the accelerator operation amount.

In step S404, the controller 72 determined the smaller one of the target input horsepower in the low vehicle speed range determined in step S402 and the target input horsepower in the middle vehicle speed range determined in step S403 as the target input horsepower to the HST 23 in the low/middle vehicle speed range.

FIG. 11 is a diagram showing processing for determining the target input horsepower to the HST 23 in the high vehicle speed range. As shown in FIG. 11, in step S501, the controller 72 determines the threshold vehicle speed in the high vehicle speed range from the accelerator operation amount and the shift position. The threshold vehicle speed in the high vehicle speed range is a vehicle speed indicative of the boundary between the low/middle vehicle speed range and the high vehicle speed range. The storage device 71 stores threshold vehicle speed data D6 that defines the relationship between the accelerator operation amount and the threshold vehicle speed. In the threshold vehicle speed data D6, the threshold vehicle speed increases according to the increase of the accelerator operation amount. The threshold vehicle speed data D6 defines the relationship between the accelerator operation amount and the threshold vehicle speed for each shift position. Although the accelerator operation amount is the same, the threshold vehicle speed increases as the shift position is higher. The controller 72 refers to the threshold vehicle speed data D6 to determine the threshold vehicle speed corresponding to the accelerator operation amount and the shift position.

In step S502, the controller 72 determines the target reference vehicle speed from the accelerator operation amount and the shift position. The controller 72 determines the target reference vehicle speed corresponding to the accelerator operation amount and the shift position with reference to the reference vehicle speed data D1 described above.

In step S503, the controller 72 determines the zero traction vehicle speed from the accelerator operation amount and the shift position. The zero traction vehicle speed means the target vehicle speed when the traction force is zero, that is, when the traveling load is zero. The storage device 71 stores zero traction vehicle speed data D7 that defines the relationship between the accelerator operation amount and the zero traction vehicle speed. In the zero traction vehicle speed data D7, the zero traction vehicle speed increases in accordance with the increase in the accelerator operation amount. The zero traction vehicle speed data D7 defines the relationship between the accelerator operation amount and the zero traction vehicle speed for each shift position. Although the accelerator operation amount is the same, the zero traction vehicle speed increases as the shift position is higher. The controller 72 determines the zero traction vehicle speed corresponding to the accelerator operation amount and the shift position with reference to the zero traction vehicle speed data D7.

When the accelerator operation amount and the shift position are the same, the threshold vehicle speed data D6, the reference vehicle speed data D1, and the zero traction vehicle speed data D7 are set such that the target reference vehicle speed is greater than the threshold vehicle speed and lower than the zero traction vehicle speed.

In step S504, the controller 72 determines the static target input horsepower to the HST 23 from the target vehicle speed. When the target vehicle speed is equal to or less than the threshold vehicle speed, the controller 72 determines the above-described target input horsepower in the low/middle vehicle speed range as the static target input horsepower.

When the target vehicle speed is the target reference vehicle speed, the controller 72 determines the target reference traveling horsepower calculated by multiplying the target reference traction force by the target reference vehicle speed as the static target input horsepower. For example, the controller determines the target reference traction force from the weight of the work vehicle 1 and a predetermined coefficient. The weight of the work vehicle 1 and the predetermined coefficient are stored in the storage device 71.

The controller 72 sets the static target input horsepower to zero when the target vehicle speed is equal to or greater than the zero traction vehicle speed. When the target vehicle speed is a value between the threshold vehicle speed and the target reference vehicle speed, or a value between the target reference vehicle speed and the zero traction vehicle speed, the controller 72 determines the static target input horsepower to the HST 23 by linear interpolation.

The static target input horsepower mentioned above is the target input horsepower to HST 23 at steady state. At the time of transition due to the change of the accelerator operation amount, the controller 72 increases the target input horsepower to the HST 23 at a speed according to the accelerator operation amount within a range not exceeding the static target input horsepower. FIG. 12 is a diagram showing processing for determining the target input horsepower (dynamic target input horsepower) to the HST 23 at the time of transition.

As shown in FIG. 12, in step S601, the controller 72 determines the amount of increase in horsepower from the target acceleration described above, the actual vehicle speed, and the transmission efficiency. The amount of increase in horsepower means the amount of increase in input horsepower to HST 23 per unit time required to increase the vehicle speed at the target acceleration from the actual vehicle speed.

In step S602, the controller 72 determines the current target input horsepower by adding the amount of increase in horsepower to the previous target input horsepower. In step S603, the controller 72 selects the larger of the current target input horsepower determined in step S602 and the above-described target input horsepower in stalling as the dynamic target input horsepower. In step S604, the controller 72 selects the smaller one of the dynamic target input horsepower determined in step S603 and the static target input horsepower described above as the target input horsepower.

As described above, the controller 72 determines the current dynamic target input horsepower by increasing the previous dynamic target input horsepower by the amount of horsepower increase corresponding to the accelerator operation amount. Then, the controller 72 increases the dynamic target input horsepower every unit time between the target input horsepower in stalling and the static target input horsepower.

Next, as shown in FIG. 6, in step S104, the controller 72 acquires the work implement operation amount. The controller 72 acquires the work implement operation amount from the signal from the work implement operation sensor 68.

In step S105, the controller 72 determines a target engine speed. The controller 72 determines the target engine speed from the target input horsepower to the HST 23 and the work implement operation amount. FIG. 13 is a diagram showing a process for determining the target engine speed.

As shown in FIG. 13, in step S701, the controller 72 determines a target engine speed for the HST 23 from the target input horsepower determined in step S604. The storage device 71 stores engine torque-rotational speed data D8 that defines the relationship between the engine torque and the target engine speed for the HST 23. The controller 72 refers to the engine torque-rotational speed data D8 to determine the target engine speed corresponding to the target input horsepower to the HST 23. The controller 72 determines the target engine speed for the HST 23 such that the engine torque and the absorption torque of the traveling pump 31 coincide at a predetermined matching point MP on the equal horsepower line corresponding to the target input horsepower.

In step S702, the controller 72 determines a target engine speed for the work implement 3 from the work implement operation amount. The storage device 71 stores target rotational speed data D9 that defines the relationship between the work implement operation amount and the target engine speed for the work implement 3. In the target rotational speed data D9, the target engine speed increases as the work implement operation amount increases. The controller 72 refers to the target rotational speed data D9 to determine the target engine speed for the work implement 3 corresponding to the work implement operation amount.

In step S703, the controller 72 determines a target engine speed for vehicle speed from the target vehicle speed. The controller 72 determines a value calculated by multiplying the target vehicle speed by a predetermined conversion factor and the minimum transmission gear ratio as the target engine speed for the vehicle speed. The predetermined conversion factor is a factor for converting the target vehicle speed into the rotational speed of the output shaft of HST. The minimum transmission gear ratio is the minimum gear ratio of HST 23.

In step S704, the controller 72 determines the target engine speed that is the maximum among the target engine speed for the HST 23, the target engine speed for the work implement 3, and the target engine speed for the vehicle speed.

Next, as shown in FIG. 6, in step S106, the controller 72 determines the target displacement of the traveling pump 31. The controller 72 determines the target displacement of the traveling pump 31 from the target vehicle speed and the target engine speed determined in step S704. Also, in step S107, the controller 72 determines the target displacement of the traveling motor 33. The controller 72 determines the target displacement of the traveling motor 33 from the target vehicle speed and the target engine speed determined in step S704.

FIG. 14A is a diagram showing a process for determining the target displacement of the traveling pump 31. As shown in FIG. 14A, in step S801, the controller 72 determines the flow rate of the traveling motor 33 from the target vehicle speed. The controller 72 determines, as the flow rate of the traveling motor 33, a value obtained by multiplying the target vehicle speed by a predetermined conversion factor and the maximum displacement of the traveling motor 33, and dividing by the volumetric efficiency of the traveling motor 33. The predetermined conversion factor is a factor for converting the target vehicle speed into the rotational speed of the output shaft of the HST 23.

In step S802, the controller 72 determines the target displacement of the traveling pump 31 from the target engine speed and the flow rate of the traveling motor 33. The controller 72 calculates a value obtained by dividing the flow rate of the traveling motor 33 by the target engine speed and the volumetric efficiency of the traveling pump 31 as the target displacement of the traveling pump 31.

FIG. 14B is a diagram showing a process for determining the target displacement of the traveling motor 33. As shown in FIG. 14B, in step S803, the controller 72 determines the rotational speed of the traveling motor 33 from the target vehicle speed. The controller 72 calculates the rotational speed of the traveling motor 33 by multiplying the target vehicle speed by a predetermined conversion factor. The predetermined conversion factor is a factor for converting the target vehicle speed into the rotational speed of the output shaft of the HST 23.

In step S804, the controller 72 determines the flow rate of the traveling pump 31 from the target engine speed and the maximum displacement of the traveling pump 31. The controller 72 calculates the flow rate of the traveling pump 31 by dividing the value obtained by multiplying the engine speed by the maximum displacement of the traveling pump 31 by the volumetric efficiency of the traveling pump 31.

In step S805, the controller 72 determines the target displacement of the traveling motor 33 from the rotational speed of the traveling motor 33 and the flow rate of the traveling pump 31. The controller 72 divides the flow rate of the traveling pump 31 by the rotational speed of the traveling motor 33 and the volumetric efficiency of the traveling motor 33 to calculate the target displacement of the traveling motor 33.

Then, as shown in FIG. 6, in step S108, the controller 72 outputs a command signal. The controller 72 outputs a command signal to the fuel injector 24 to drive the engine 21 at the target engine speed. The controller 72 outputs a command signal to the pump displacement control device 45 to drive the traveling pump 31 at the target displacement. The controller 72 outputs a command signal to the motor displacement control device 35 to drive the traveling motor 33 at the target displacement.

In the present embodiment, when the engine 21 is in the over-rotation state, the controller 72 performs control to suppress the over-rotation of the engine 21 by limiting the target displacement of the traveling pump 31 to a predetermined upper limit value or less. Hereinafter, control for suppressing the over-rotation of the engine 21 will be described.

FIG. 15 is a flowchart showing a process executed by the controller 72 to suppress the over-rotation of the engine 21. In step S901, the controller 72 acquires the engine speed. The controller 72 obtains the engine speed by a signal from the engine speed sensor 25.

In step S902, the controller 72 acquires the HST differential pressure. The controller 72 acquires the HST differential pressure based on the signals from the first circuit pressure sensor 34 a and the second circuit pressure sensor 34 b.

In step S903, the controller 72 determines whether the engine 21 is in the over-rotation state. The controller 72 determines that the engine 21 is in the over-rotation state when the engine speed is equal to or higher than a predetermined first threshold Nth1. If the engine 21 is in the over-rotation state, the process proceeds to step S904.

In step S904, the controller 72 determines the upper limit value of the displacement of the traveling pump 31. The controller 72 determines the upper limit value of the displacement of the traveling pump 31 by the following equation (1). D_Limit=T_Limit×(2π)/(dP×et)  (1) D_Limit is an upper limit value of the displacement of the traveling pump 31. T_Limit is an allowable value of engine brake torque (hereinafter referred to as “torque allowable value”). dP is an absolute value of HST differential pressure. et is the torque efficiency of the traveling pump 31. The torque allowable value T_Limit and the torque efficiency et are constants, and are stored in the storage device 71. The torque efficiency et may be a variable. As shown by the equation (1), the controller 72 determines the upper limit value D_Limit so that the engine reverse drive torque transmitted from the traveling pump 31 to the engine 21 does not exceed the torque allowable value T_Limit.

FIG. 16 is a diagram showing the relationship between the engine speed and the engine brake torque. As shown in FIG. 16, the engine brake torque increases with an increase in engine speed. The torque allowable value T_Limit is a value of the engine brake torque when the engine speed is a second threshold Nth2. The second threshold Nth2 is larger than the first threshold Nth1 described above. The second threshold Nth2 may be the same value as the first threshold Nth1.

In step S905, the controller 72 determines whether the target displacement of the traveling pump 31 determined in S802 is larger than the upper limit value D_Limit. If the target displacement is larger than the upper limit value D_Limit, the process proceeds to step S906.

In step S906, the controller 72 determines the upper limit value D_Limit as the target displacement of the traveling pump 31. When the target displacement is equal to or less than the upper limit value D_Limit, the controller 72 maintains the target displacement of the traveling pump 31 determined in step S802. Then, in step S108 described above, the controller 72 outputs a command signal to the pump displacement control device 45 to drive the traveling pump 31 at the target displacement.

As described above, when the engine 21 is in the over-rotation state, the controller 72 limits the displacement of the traveling pump 31 to the upper limit value D_Limit or less. The upper limit value D_Limit is equal to or less than the maximum displacement of the traveling pump 31. Therefore, the controller 72 reduces the upper limit value of the displacement of the traveling pump 31 to a value smaller than the maximum displacement when the engine 21 is in the over-rotation state.

Further, as shown by the above equation (1), the upper limit value D_Limit is changed according to the HST differential pressure. Specifically, the controller 72 decreases the upper limit value D_Limit in response to the increase of the HST differential pressure.

In the work vehicle according to the present embodiment described above, the upper limit value D_Limit of the displacement of the traveling pump 31 is determined according to the HST differential pressure, and when the engine 21 is in the over-rotation state, the displacement of the traveling pump 31 is limited not to exceed the upper limit D_Limit. The engine reverse drive torque transmitted from the traveling pump 31 to the engine 21 increases as the displacement of the traveling pump 31 increases. Therefore, the engine reverse drive torque can be reduced by limiting the displacement of the traveling pump 31 to the upper limit value D_Limit or less. Thereby, an increase in engine speed can be suppressed to prevent over-rotation.

Further, the engine reverse drive torque transmitted from the traveling pump 31 to the engine 21 changes in accordance with the HST differential pressure. Therefore, the engine reverse drive torque can be appropriately suppressed by determining the upper limit value of the displacement of the traveling pump 31 according to the HST differential pressure. Further, since the reduction of the transmission ratio of HST 23 is suppressed, the reduction of the braking force due to the engine brake can be suppressed.

Preferred embodiments of the present invention have been explained above. However, the present invention is not limited to the aforementioned preferred embodiments, and a variety of changes can be made without departing from the scope of the present invention.

The work vehicle 1 is not limited to a wheel loader, and may be another type of vehicle such as a motor grader. The configurations of the drive system and control system of the work vehicle 1 are not limited to those of the above embodiment, and may be changed. For example, the displacement of the traveling pump 31 may be controlled by another control valve, not limited to the pump control valve 47. That is, a control valve for controlling the pressure of the hydraulic fluid supplied to the pump control cylinder 46 via the pump pilot circuit 48 may be provided separately from the pump control valve 47.

The parameters used for the various operations described above are not limited to those described above, and may be changed. Alternatively, parameters other than the above-described parameters may be used for the calculation. The various data described above may be represented, for example, by an equation, or may be in the form of a table, a map, or the like. The upper limit of the displacement of the traveling pump 31 is not limited to the equation, and may be determined from a table or a map.

The order of the processes described above may be changed. Alternatively, some processing may be executed in parallel. For example, step S101 and step S104 may be executed in parallel.

The controller 72 may determine the target vehicle speed by a method different from the above embodiment. The controller 72 may determine the target input horsepower to the HST 23 by a method different from the above embodiment. The controller 72 may determine the target engine speed by a method different from the above embodiment. The controller 72 may determine the target displacement of the traveling pump 31 by a method different from the above embodiment. The controller 72 may determine the target displacement of the traveling motor 33 by a method different from the above embodiment. Also in such a case, when the engine 21 is in the over-rotation state, the over-rotation can be suppressed by limiting the displacement of the traveling pump 31 to the upper limit value or less as described above.

According to the present invention, it is possible to prevent over-rotation of the engine speed and to suppress a decrease in braking force due to the engine brake. 

The invention claimed is:
 1. A work vehicle comprising: an engine; a traveling pump driven by the engine; a hydraulic circuit connected to the traveling pump; a traveling motor connected to the traveling pump via the hydraulic circuit; a rotational speed sensor configured to output a signal indicative of engine speed; a hydraulic pressure sensor configured to output a signal indicative of hydraulic pressure of the hydraulic circuit; and a controller configured to receive a signal from the rotational speed sensor and the hydraulic pressure sensor, determine whether the engine is in an over-rotation state based on the engine speed, the controller determining that the engine is in the over-rotation state when the engine speed is equal to or greater than a predetermined first threshold, determine an upper limit value of displacement of the traveling pump based on the hydraulic pressure of the hydraulic circuit, the upper limit value being determined such that an engine reverse drive torque transmitted from the traveling pump to the engine does not exceed a torque allowable value, the torque allowable value being a value of engine brake torque when the engine speed is a second threshold equal to or greater than the first threshold, and limit the displacement of the traveling pump to the upper limit value or less when the engine is in the over-rotation state.
 2. The work vehicle according to claim 1, wherein the controller is further configured to decrease the upper limit value when the engine is in the over-rotation state.
 3. The work vehicle according to claim 1, wherein the controller is further configured to decrease the upper limit value in response to an increase in the hydraulic pressure of the hydraulic circuit.
 4. A method executed by a controller in order to control a work vehicle including an engine, a traveling pump driven by the engine, a hydraulic circuit connected to the traveling pump, and a traveling motor connected to the traveling pump via the hydraulic circuit, the method comprising: receiving a signal indicative of engine speed; receiving a signal indicative of hydraulic pressure of the hydraulic circuit; determining whether the engine is in an over-rotation state based on the engine speed, the engine being in the over-rotation state when the engine speed is equal to or greater than a predetermined first threshold; determining an upper limit value of a displacement of the traveling pump based on the hydraulic pressure of the hydraulic circuit, the upper limit value being determined such that an engine reverse drive torque transmitted from the traveling pump to the engine does not exceed a torque allowable value, the torque allowable value being a value of engine brake torque when the engine speed is a second threshold equal to or greater than the first threshold; and limiting the displacement of the traveling pump to the upper limit value or less when the engine is in the over-rotation state. 